Papilloma viruses infect a range of hosts including man, cattle, sheep, dogs and cats. For a more complete listing, see "Papilloma Virus Infections in Animals" by J. P. Sundberg which is described in Papilloma Viruses and Human Diseases, edited by K. Syrjanen, L. Gissman and L. G. Koss, Springer Verlag, 1987.
Human papilloma viruses induce benign hyperproliferative lesions of the cutaneous and mucosal epithelia. Of the 70 different virus types which infect humans, more than 20 are associated with anogenital lesions (de Villiers, 1989, J. Virol. 63 4898-4903). Papilloma viruses have also been associated with various forms of cancers. Human papilloma virus types 16 and 18 have been associated with a number of cervical intra-epithelial neoplasias and carcinomas of the cervix (Lancaster et al., 1987, Cancer Metast. Rev. 6 6653-6664 and Pfister, 1987, Adv. Cancer Res. 48 113-147).
Papilloma viruses are small DNA viruses encoding up to eight early and two late genes. The late genes L1 and L2 code for structural proteins which assemble into a capsid within the cell (Galloway et al., 1989, Adv. Virus Res. 37 125-171). A single virus capsid is a T=7d icosahedron composed of 360 pentameric capsomers, each of which contains five molecules of the major capsid protein L1 (Baker et al., 1991, Biophys. J. 60 1445-1456 and Finch et al., 1965, J. Mol. Bio. 13 1-12). The minor capsid protein L2 is present at approximately one-tenth the abundance of L1 (Doorbar et al., 1987, J. Virol. 61 2793-2799).
Propagation of human papilloma viruses in vitro has not been achieved (Taichman et al., 1984, J. Invest. Dermatol. 83 25) and only small amounts of HPV proteins have been isolated from infected tissues (Androphy etal., 1987, Embo J. 6 1989; Banks et al., 1987, J. Gen. Virol. 68 1351; Firzlaff et al., 1988, Virology 164 467; Oltersdorf et al., 1987, J. Gen. Virol. 68 2933; Schneider-Gadicke et al., 1988, Cancer Res. 48 2969; Seedorf et al., Embo J. 6 139 and Smotkin et al., 1986, PNAS 83 4680). However, the gene coding for L1 protein has been cloned and expressed in a eukaryotic expression system using recombinant vaccinia virus (Browne etal., 1988, J. Gen. Virol. 69 1263-1273; Zhou et al., 1990, J. Gen. Virol. 71 2185-2190 and Zhou et al., 1991, Virology 185 251-257), in a baculovirus expression system (Park et al., 1993, J. Virol. Meth. 45 303-318) and in a bacterial expression system (Strike et al., 1989, J. Gen. Virol. 70 543-555).
As L1 protein is the major capsid protein, it has been used as the basis for the development of vaccines for protection against papilloma virus infection. Zhou et al. immunized mice with synthetic HPV16 virus-like particles (VLPs) using a vaccinia virus doubly recombinant for the L1 and L2 proteins of HPV16. The murine anti-VLP anti-sera recognised HPV16 capsids by ELISA and baculovirus recombinant HPV16L1 and L2 protein on immunoblot. The murine anti-VLP anti-sera, however, failed to recognise two peptides that were recognised by anti-HPV16L1 monoclonal antibodies raised against a recombinant L1 fusion protein (Zhou et al., 1992, Virology 189 592-599). These researchers concluded that the immunoreactive epitopes of HPV16 defined using virus-like particles differ significantly from those defined using recombinant HPV16L1 fusion proteins.
To overcome problems of presentation, vaccines were developed using virus-like particles. VLPs were formed intracellularly from recombinant L1 or L1 and L2 proteins encoded by recombinant vaccinia virus (Zhou et al., 1991, Virology 185 251-257; Zhou et al., 1991, Virology 181 203-210 and International Patent Specification WO93/02184). These vaccines using synthetic virus-like particles have a number of disadvantages. Firstly, the recombinant L1 or L1 and L2 genes are expressed from a vaccinia virus vector which may not be suitable for the production of a vaccine. Secondly, the virus-like particles are produced intracellularly which is a rate limiting step. Thirdly, the virus-like particles may incorporate cellular DNA because they are produced intracellularly and virus-like particles incorporating DNA are not suitable for use in vaccines. Fourthly, virus-like particles may only be partially purified because of the need to retain their integrity and hence correct epitope presentation. Consequently, other proteins or matter associated with the virus-like particles may contaminate a vaccine preparation. Fifthly, the process of producing a vaccine in commercial amounts with virus-like particles from recombinant vaccinia viruses is comparatively expensive.
Similar disadvantages apply to the use of the virus-line particles produced from recombinant vaccinia viruses for the detection of antibodies in the sera of patients.